Solid-state electrochemical devices are normally cells that include two porous electrodes, the anode and the cathode, and a dense solid electrolyte membrane disposed between the electrodes. In the case of a typical solid oxide fuel cell, the anode is exposed to fuel and the cathode is exposed to an oxidant in separate closed systems to avoid any mixing of the fuel and oxidants due to the exothermic reactions that can take place with hydrogen fuel.
The electrolyte membrane is normally composed of a ceramic oxygen ion conductor in solid oxide fuel cell applications. In other implementations, such as gas separation devices, the solid membrane may be composed of a mixed ionic electronic conducting material (“MIEC”). The porous anode may be a layer of a ceramic, a metal or a ceramic-metal composite (“cermet”) that is in contact with the electrolyte membrane on the fuel side of the cell. The porous cathode is typically a layer of a mixed ionically and electronically-conductive (MIEC) metal oxide or a mixture of an electronically conductive metal oxide (or MIEC metal oxide) and an ionically conductive metal oxide.
Solid oxide fuel cells normally operate at temperatures between about 900° C. and about 1000° C. to maximize the ionic conductivity of the electrolyte membrane. At appropriate temperatures, the oxygen ions easily migrate through the crystal lattice of the electrolyte.
Since each fuel cell generates a relatively small voltage, several fuel cells may be associated to increase the capacity of the system. Such arrays or stacks generally have a tubular or planar design. Planar designs typically have a planar anode-electrolyte-cathode deposited on a conductive interconnect and stacked in series. However, planar designs are generally recognized as having significant safety and reliability concerns due to the complexity of sealing of the units and manifolding a planar stack. Tubular designs utilizing long porous support tubes with electrodes and electrolyte layers disposed on the support tube reduce the number of seals that are required in the system. Fuel or oxidants are directed through the channels in the tube or around the exterior of the tube.
The utility of high-temperature (e.g., greater than 800° C., for example between about 900 and 1000° C.) electrochemical devices is limited by the quality and robustness of the seals that join multiple cells together or individual cells to cell housings or manifolds. Seals generally need to provide one or more of the following functions: separation of oxidant/fuel/process gases from each other and containment of the gases within the device, bonding between the sealed surfaces, and electrical connection or insulation. Of course, the seal material must not be a source of contamination for the other materials in the system. It is difficult for a single material to perform all of these functions at elevated temperature in oxidizing, reducing, or corrosive environments.
Many types of seal materials have been considered for use in high-temperature electrochemical devices, including ceramic adhesives, glass, brazes, and mica compressive seals. Each of these materials has limitations that prevent it from fulfilling all of the necessary requirements. Ceramic adhesives tend to be porous, preventing gas sealing. Glasses provide a good initial seal, but have a short lifetime due to thermal stress-induced cracking and chemical reaction with the joined surfaces. Brazes are expensive and conductive. Mica compressive seals often have a high leak rate and short lifetime due to cracking.
Thus, an improved sealed joint for electrochemical devices is needed.